Extrusion process for making thermoplastic tubular film



Jan. 25, 1966 GOLDMAN 3,231,652

EXTRUSION PROCESS FOR MAKING THERMOPLASTIC TUBULAR FILM Filed July 9, 1964 FIG.1

COOLANT 22 COOLANT OUT INVENTOR MAX GOLD MA N BY MM ATTORNEY United States Patent Cfifice 3,231,652 Faten ted Jan. 25, 1966 3,231,652 EXTRUSION PROCESS FOR MAKING THERMOPLASTIC TUBULAR FILM Max Goldman, Tonawanda, N.Y., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed July 9, 1964, Ser. No. 381,306 Claims. (Cl. 264-269) This application is a continuation-in-part of copending application Serial No. 51,629, filed August 24, 1960, and now U.S. Patent No. 3,141,912.

This invention relates to the Orientation of thermoplastic polymeric film in tubular form. More particularly, it relates to a method for orienting by a combination of expansion and longitudinal stretching to improve the properties of thermoplastic, polymeric, tubular film, and still more particularly, to tubular feed rate control in the production of thermoplastic, polymeric film.

The idea of orienting under controlled conditions, particularly at a controlled temperature, to improve the physical properties of thermoplastic polymeric film, is not new. The idea of expanding a thermoplastic, polymeric, tubular film by providing gas under pressure within the tubular film is not new either. The problem, however, of maintaining control over the temperature of the tubular film prior to and during expansion of the film has also prevailed for a long time and is a problem that has discouraged the use of the expansion or blowing process for the orientation of polymeric film.

Also, improvements in the extrusion and orientation of thermoplastic, polymeric films, wherein internal guides or heaters extending into the film tube and bubble are indicated, has made it necessary to remove the intermediate collapsing rolls between the extrusion and orientation zones and has created a need for a process in which the intermediate collapsing rolls are not used.

'It is an object of the present invention to provide a process for orienting thermoplastic, polymeric, tubular film.

It is a further object of this invention to provide in the production of oriented thermoplastic, polymeric, tubular film a process for tubular feed rate control in which the intermediate collapsing rolls are eliminated. These and other objects will appear hereinafter.

The manner in which the objects of the invention are attained is set out in the following description and the drawing, in which:

FIGURE 1 is a schematic view, partially in section, of an apparatus adapted to carry out the process of the present invention; and,

FIGURE 2 is a partial schematic View showing end less belts used as the tube advancer.

The objects are accomplished by a process which involves, first extruding thermoplastic polymeric material in the form of a tubular sheet or film heated to thermoplastic condition, i.e., in a formative state above its crystalline melting point; quenching the tubular sheet by cooling the sheet quickly to a temperature below the formative state, preferably at least 20 C. below the orientation temperature range; advancing the tubular sheet at a predetermined initial rate with speed controlled moving surfaces which surround at least partially and grip the tubular sheet over a portion of its length without flattening it to feed it positively; maintaining sufiicient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion of at least 2 times the original diameter of the tubular sheet; heating is preferably accomplished by heating the tubular sheet to a temperature just below the orientation temperature range but no less than a temperature of preferably of the lowest temperature of the orientation temperature range in degrees centigrade; heating the tubular sheet internally from an internally disposed radiant heat source to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times, preferably 210 times its original diameter as extruded; advancing while expanding the heated tubular sheet at a rate at least 2 times, preferably 2-10 times the initial rate, preferably the ratio of the final advancing rate to the initial advancing rate being equal to the ratio of the final diameter to the extruded diameter of the tubular sheet; and, finally, cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

The orientation temperature range, as defined herein, refers to the temperature range in which molecular orientation of a polymeric film may be eifected. This range lies somewhere below the melting temperature of a polymer that melts at a specific temperature or below the crystalline melting point of a crystalline polymer that melts over a range of temperatures. The crystalline melting point refers to the temperature at which the crystallites of a crystalline polymer are no longer detectable under X-ray examination when the solid polymer is heated until it melts.

For some crystalline polymers such as polyethylene, polypropylene and other polyhydrocarbons, the orientation temperature range may be the range of temperature over which the crystallites melt but below the temperature at which the crystallites are no longer detectable. In the case of polyesters such as polyethylene terephthalate and the like, the socalled crystallizable polymers, the orientation temperature range ext-ends from about 10 C. to 40 C. above the second order transition temperature of the polymer. The second order transition temperature is that temperature at which an essentially amorphous polymer or one that can be quenched as an amorphous polymer but is crystallizable makes a transition from a glassy state to a rubbery state. It is in this rubbery state that the polymer in the form of a film or a filament can be oriented by stretching. The second order transition temperature varies with the molecular weight of the polymer and is defined more completely in US, Patent 2,578,899.

The specific orientation temperature range will vary from polymer to polymer but may be determined by experimentation or from the literature. In the following table, Table I, the orientation temperature range, the second order transition temperature and the crystalline melting point are given for some representative amorphous polymers and some crystallizable polymers that are amorphous as quenched.

TABLE I Second Crystal- Orienta,

Order line tion Polymer Transi- Melting Temperation Point ture Temp. 0.) Range Polyethylene terephthalate 70 255 85-110 Polyethylene-2,6-naphthalate 113 265 120-140 Polytetramethylene-l,2-dioxybenzoate 53 220 70-90 Polyethylene-1,5-naphthalate. 71 225 80-100 Polyhexamethylene adipamide 45-50 250 65-75 Polyhexamethylene sebacarnide 45-50 250 65-75 Polycaprolactam 45-50 250 65-75 70% Ethylene terephthalate/30% eth- 51 170 70-90 lene isophthalate copolymer Polyvinyl chloride:

N0 plasticizer 105 170 115-145 plasticizen... 90 170 100-130 plasticizen. 75 170 85-115 plastieizer 60 170 70-100 84% tetrafluoroethylene/16% hexafluoropropylene copolymer. 85 275 95-125 Polystyrene None 1 88-120 88-110 Polyrnethylrnethacrylate None 1 66-111 06-105 1 Softening range rather than crystalline melting point since the polymer is only obtainable as an amorphous polymer.

In the following table, Table II, the orientation temperature range and the crystalline melting point are listed for some representative crystalline polymers.

In the following table, Table III, the preferred conditions of this process for several particularly important polymers are listed.

TABLE III Quench Temp. in Temp. in Elongation Temp. Initial Final Range Polymer Range Heating Heating (times C.) Zone C.) Zone 0.) original dimensions) Polypropylene 0-40 130-145 150-159 2-10 Polyethylene terephthalate -50 60-80 85-110 2-6 Polyethylene 2 0-40 80-120 100-130 2-10 Polyvinyl chloride 3 20-40 65-80 85-115 2-6 1 Density above 0.909 gms/ce. 2 Density above 0.92 gms./cc. 3 Plastieizer content of 0-15%.

Referring to the figure, which is a diagrammatic illustration of the process of this invention, the thermoplastic polymer is first heated to a temperature above its melting point in the extruder 10. The molten polymer, preferably at a temperature at least 10 C. above its melting point or crystalline melting point, is extruded through die 11 in the form of a tubular film 14, the tube having a wall thickness of anywhere from 15 to 85 mils. The tubular film is then drawn over a cooling mandrel 12 by means of the tube advancer 13. The tube advancer 13 is composed of two sets of driven rubber squeeze rolls which contact the surface of the cooled tubular film to advance the film at a predetermined initial rate. The tube advancer 13 also serves to prevent sway of the film and to insulate the freshly extruded film adjacent the die from the subsequently applied longitudinal tension. Instead of driven rolls, endless belts may be used as the tube advancer.

Tube advancer 13 or feed control means constructed to carry out tubular feed rate control process is defined as comprising at least two moving surfaces constructed to surround at least partially and grip a tube over part of its length but not to scratch, flatten, mar or permanently distort it, and a speed control operatively connected with the moving surfaces to control their speed so that a tube may positively be fed by the feed control means at a controlled rate. Preferably the parts of the moving surfaces which contact the tube are of a yieldable material, such as rubber.

The speed control preferably comprises external driving or driven means which is positively connected to the moving surfaces, for instance a speed controlled electric motor which is preferably coupled through worm reduction gearing. When the process is in operation, the longitudinal stretching force applied to the tube will urge along the moving surfaces and such speed controlled electric motor will be acting as a brake. On start up the motor will supply power to feed the tube. The speed control may however be a brake which is adjustable to provide the desired control.

Preferably, the moving surfaces comprise at least two endless belts constructed and arranged to present longitudinal gripping surfaces for the tube. To get an increased friction between the tube and such endless belts without unduly distorting the tube, it is preferred to provide each endless belt with a longitudinal channel into which the tube may be fitted. Naturally however each such channelled belt may be replaced by two or more independent belts (i.e., independent in the sense that they are not parts of a single belt). A further preference is that the parts of the endless belts which contact the tube consist of foam rubber.

Alternatively, the moving surfaces comprise a number of rollers, the number, construction and arrangement of which is such that the tube may be fed positively by them (i.e., without slipping). Preferably each roller is cylindrical so that it has substantially the same angular velocity along its length when not greatly compressed. To increase grip, it may be surfaced with foam rubber.

The cooling or quench mandrel 12 is a hollow metal cylinder insulated from the die by a non-metallic insulating disc 15. The disc has openings 26 around its periphery, the openings communicating With the atmosphere through conduit 27. Coils 30 communicating with inlet and outlet tubes 28 and 29 in the conduit, are disposed adjacent to the interior surface of the mandrel. The coils 30 convey cooling water which serves to reduce the temperature of the tubular film to about room temperature, although any temperature from -l0 C. to about 50 C. would be adequate to convert most thermoplastic polymeric films to a non-formative plastic state, i.e. a state where the film resists stretching. Since cooling tends to shrink the thermoplastic polymeric tubular film, the mandrel is preferably tapered to the contour of the shrinking film. If necessary, pressure relief areas such as those disclosed in U.S. Patent 2,987,765 may be provided to isolate the gas pressure downstream of the mandrel 12.

The solidified tubular film is then drawn through 'a guide ring 16, the ring serving to minimize sway of the tube. Drawing is accomplished by means of a set of nip rolls 24 rotating at a rate that is at least 2 times the rate of the tube advancer 13. Next, the film is advanced through the initial heating section 19. This section is composed of the forced air heaters 17 and the radiant heater 18 connected to a power source, not shown. The connecting wires are also brought in through the conduit 27. It should be understood that any other means that would serve to heat the outside surface of the tubular film would operate in this invention. The initial heating section 19 serves to heat the film to a temperature within about 70% of the lowest temperature of the orientation temperature range to just below the orientation temperature range of the thermoplastic polymer. The precise temperature to which the film is heated in the initial heating section 19 will depend upon several factors. For example, the greater the speed of the film, the higher the temperature to which the film must be heated. The greater the intensity of the final internal heater 22, the lower the temperature to which the fihn need be'heated in the initial heating section 19.

Air or other gaseous medium admitted through inlet 20 and vented through outlet 21 provides the pressure within the tubular film to prevent collapse of the film while the film is at a temperature below the orientation temperature range and to expand the tubular film a predetermined amount when the temperature of the film reaches the orientation temperature range. This circulating air also serves to cool the inside surface of the tubular film. The amount of expansion is dependent upon the pressure within the film, the precise temperature within the orientation t-emperature range to which the film is brought, the rate of heating the film, the thickness of the film wall, etc., and can either be pre-set by one skilled in the art or set after experimentation.

The tubular film is brought to a temperature within the orientation temperature range and at which the film expands by means of the pencil-type, internal radiant heator 22 mounted on the internal guide ring 23. Heater 22 is connected to a power source by wires leading through the conduit 27 to a power source, not shown. The tubular film 14, immediately upon reaching the orientation temperature range, starts to expand due to the pressure within it and to elongate due to the relative rates of the nip rolls 24 and the tube advancer 13. However, as the tubular film expands, its inner surface gets farther and farther away from heater 22 and its outside surface approaches the cooling ring 25. This combination of factors serves to reduce the temperature of the film. The resulting expanded and stretched tubular film is then cooled by con- .tinuing its passage through the cooling ring 25. Cooling ring 25 can contain coils to convey cooling water in its surface as in the quenching mandrel. It should be understood that neither the precise heating means nor the precise cooling means described or shown in the figure are to be construed as limitative in the present invention.

By adhering to the limits of the process of the present invention, several surprising results are at once apparent. The resulting biaxially oriented film does not display the usual magnified gauge variations normally accompanying film-blowing processes. That is, gauge variations, almost unavoidable during extrusion, are not magnified as in prior art blowing processes by the expansion process of .the present invention. Another unexpected result is that polypropylene and other crystalline polymers which heretofore had suffered from line drawing when stretched by expanding the tubular film, do not suffer from this fatal shortcoming when subjected to the process of the present invention.

A further advantage of the invention arises from the lack of the creases made in the tube by take-oil? nip rolls. Such creases cause weaknesses in and may disfigure the stretched film. By following this invention layflat film can be obtained free from weaknesses due to such creasing in the take-oif nip rolls, which layflat film may slit down one edge only and the tube opened out to a flat film. In this way the size of the inflated tube of film may be halved for the same width of fiat film produced (compared with the case in which layflat film is slit at both edges because weakness at the edges due to the creasing is caused by the take-off nip rollers).

The following examples serve to illustrate the present invention, Example 1 being the best mode contemplated for practicing the invention.

Example 1 Molten polypropylene having a density of 0.906 1 was extruded at a temperature of 225 C. and at a rate of 10 pounds per hour from a l-inch extruder through a 2% inch circular die having a 45-mil lip opening. The apparatus was substantially that shown in the drawing. The extruded tubular film was quenched to the non-formative plastic state by being passed over a nominal 2-inch tapered internal mandrel at a temperature of about 10 C., the taper of the mandrel being 4 mils per inch. The tubular film was withdrawn from the die over the internal mandrel by a tube advancer operating at 2.5 feet per minute. This advancer comprises grooved, foam rubber covered by hard rubber rollers and are driven by a worm drive and gear box (not shown) and a speed controlled electric motor (not shown). Air was injected and vented to the atmosphere through connections in the end of the quench mandrel. The resulting open bubble system provided a pressure of 22 inches of water within the tubular film. As the tubular film cooled over the internal mandrel, it shrank in diameter. This shrinkage, together with the taper of the mandrel, served to prevent the air injected at the end of the mandrel from exerting pressure on the molten tubular film in the area adjacent the die.

After passing through the tube advancer, the tubular film was led through an external tapered guide ring 6 inches long and 2 inches in diameter to minimize sway. The tubular film was then passed into an initial heating Zone where it was heated to a temperature of C. The initial heating zone was composed of several external radiant heaters that surrounded the film and a forced air heater. The radiant heaters were each 12 inches high, 10 inches in diameter, rated at 650.0 watts and operated at 25% capacity. The forced air heater, which was located just upstream of the radiant heaters, forced air at a temperature of about 160 C. and at a rate of about 1,200 get per minute against the outside surface of the tubular Immediately after passing through the initial heating zone, the tubular film was passed into the final heating zone where it was heated to a temperature of about C. Heating was provided by a pencil-type internal radiant heater 7 inches long, rated at 650 watts, operated at 72% capacity and having a surface temperature of 620 C. At the internal pressure of 22 inches of water and at the temperature of 155 C., the tubular film stretched and expanded. Stretching of six times its original length was accomplished by a set of nip rolls that drew the film at a rate of 15 feet per minute. Expansion to a diameter of 12 inches took place due to the internal pressure of 22 inches of water at this temperature.

The major amount of cooling was accomplished by passing the expanded film through .a water-cooled ring 10 /2 inches long and 12 /2 inches in diameter which served to cool the expanded and stretched film to a temperature of about 40 C. The tubular film which had been elongated six times in both the transverse and longitudinal directions was finally collapsed as it passed through the nip of a set of nip rolls and was finally wound on a roll.

The film having a thickness of 0.8 mil displayed the following properties:

1 In the direction of movement of the film through the apparatus. 2 Perpendicular to the longitudinal direction. 3 Independent of direction.

1 Pro-fax 651210.

and suspended in boiling Water for 1 second. The dimensional change is then noted, and percent shrinkage is calculated based on the original dimension.

Examples 2-5 Example 1 was repeated using different polymeric materials in substantially similar equipment as that shown in Example 1. The particular polymeric materials and the specific operating conditions are summarized in modulus in p.s.i. is directly related to the film stiffness. 10 Table IV.

TABLE IV Example Polymer Kopper 75% Esso Super Alathon 14 Polyvinyl (266C) Dylan 25 Chloride 4 Extrusion Temp. C.) 225 215 215 190 Quench Temp. 0.). 25 25 25 25 Initial Diameter (inches) 2 2 2 2 Initial Rate of Drawing (it/min.) 2.6 2. 2. 5 2.5 Temp. of Film in Initial Heating Zone 125 115 100 80 Surface Temp. oi Exte al Heater C.) 250 280 320 200 Bubble Pressure (inches of Water) 24 8 6 8 Temp. of Film in Final Heating Zone C. 150 125 115 90 Surface Temp. of Internal Radiant Heater 0.). 610 570 350 700 Final Diameter (inches). 10 12 10 4 Final Rate of Drawing (f 13 14. 5 12. 5 8 Amount of LD Stretch 5X 6X 5X 3. 2X Amount of TD Stretch 5X 6X 5X 2. 0X

1 Linear polypropylene having a density of 0.902. 2 Linear polyethylene having a density of 0.954

a 75% of polyet hylene having a density of 0.915 and oi polyethylene having a density pinene.

It is obtained from the slope of the stress-strain curve at an elongation of 1%. Both tensile strength and initial tensile modulus are based upon the initial cross-sectional area of the sample.

H Pneumatic Impact Strength or Impact Strength is the energy required to rupture a film. It is reported in kilograms-centimeters per mil thickness of the sample. The pneumatic impact strength is determined by measuring the velocity of a ball mechanically accelerated by air pressure, first in free flight and then in a flight impeded by rupturing the test sample. In this test, the film sample is 1% x 1%". The projectiles are steel balls /2 in diameter and weighing 8.3 grams. The free flight velocity is 23 meters per second. The velocities are measured by timing photoelectrically the passage of the steel balls between two light beams set a definite distance apart. The pneumatic impact strength is measured by the loss in kinetic energy due to the rupturing of the test sample. It is calculated from the following formula:

Constantx (Square of velocity in free flight-Square of velocity in impeded flight) Where the constant is directly proportional to the weight of the projectile and inversely proportional to the acceleration due to gravity. This test is carried out at 23 C. and 50% relative humidity and the test samples are conditioned for 24 hours at 23 C. and 50% relative humidity.

Strinkage is a measure of the form-stability of the film. Ten sample strips, 2.5 x 5" each, from each direction, i.e. ten having the longer dimension running in the machine or longitudinal direction or the direction in which the film was extruded and ten having the longer dimension running in a direction transverse to the machine direction, a re supported from two adjacent corners The thickness and the physical properties of the films resulting in Examples 25 are presented in Table V.

What is claimed is:

1. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; quenching the tubular sheet to a temperature below its formative state; advancing the tubular sheet at a predetermined initial rate with speed controlled moving surfaces Which surround at least partially and grip the tubular sheet over a portion of its length without flattening it to feed it positively; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the orig inal diameter of the sheet; heating the tubular sheet to a temperature within its orientation range; advancing, While expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cool! ing the tubular sheet while maintaining the sheet sub= stantially at its expanded diameter.

2. The process of claim 1 wherein the thermoplastic polymeric material is polypropylene.

3. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; quenching the tubular sheet to a temperature below its formative state; advancing the tubular sheet at a predetermined initial rate with speed controlled moving surfaces of external driven rollers which surround at least partially and grip the tubular sheet over a portion of its length without flattening it to feed it positively; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the tubular sheet to a temperature between 70% of the lowest temperature of the orientation temperature range in degrees centigrade and just below the orientation temperature range of the polymeric material; thereafter, heating the tubular sheet internally from an internally disposed radiant heat source to a temperature within the orientation temperature range to expand the tubular 1% sheet to a diameter at least 2 times its original diameter, said expansion being carried out while said tubular sheet is exposed to said heat source; advancing while expanding, the heated tubular sheet at a rate of at least 2 times the initial rate of advancement; and cooling the tubular sheet substantially at its expanded diameter.

4. The process of claim 3 wherein the tubular sheet is advanced with speed controlled moving surfaces of external driven endless belts.

5. The process of claim 3 wherein the thermoplastic polymeric material is polypropylene.

References (Iited by the Examiner UNITED STATES PATENTS 2,541,064 2/1951 Irons 264-95 2,634,459 4/ 1953 Irons 26495 2,916,764 12/1959 Garber 26495 3,141,912 7/1964 Goldman et al 264-209 ALEXANDER H. BRODMERKEL, Primary Examiner.

ROBERT F. WHITE, Examiner. 

1. THE PROCESS COMPRISING: EXTRUDING THERMOPLASTIC POLYMERIC MATERIAL IN THE FORM OF A TUBULAR SHEET IN ITS FORMATIVE STATE; QUENCHING THE TUBULAR SHEET TO A TEMPERATURE BELOW ITS FORMATIVE STATE; ADVANCING THE TUBULAR SHEET AT A PREDETERMINED INITIAL RATE WITH SPEED CONTROLLED MOVING SURFACES WHICH SURROUND AT LEAST PARTIALLY AND GRIP THE TUBULAR SHEET OVER A PORTION OF ITS LENGTH WITHOUT FLATTENING IT TO FEED IT POSITIVELY; MAINTAINING SUFFICIENT PRESSURE WITHIN THE TUBULAR SHEET TO PREVENT COLLAPSE AND TO PROVIDE, WHEN THE TUBULAR SHEET IS HEATED TO A TEMPERATURE WITHIN ITS ORIENTATION TEMPERATURE RANGE, AN EXPANSION TO A DIAMETER OF AT LEAST 2 TIMES THE ORIG- 